1. Field of the Invention
The present invention relates to an image pickup technology, and more particularly to an image pickup apparatus and an image pickup method for clamping an optical black level to a predetermined level.
2. Description of the Related Art
Conventionally, in displaying and recording a still image and a motion picture picked up by a solid-state imaging device such as a CCD, an optical black (hereafter referred to as OB) level of the imaging device is utilized as a reference of luminance level of an image signal. OB refers to a pixel output shaded within a photo detection pixel portion of the imaging device and not dependant on an incident light.
One example of a device configuration of a feedback clamp DC component recovery (clamp) circuit using the OB level as a black reference of the image signal, which is commonly utilized in a video camera and the like, is shown in FIG. 7. A main operating waveform thereof is shown in FIG. 8, to briefly explain the operation.
In FIG. 7, first, a solid-state imaging device 101 such as a CCD is provided. The image signal outputted from the solid-state imaging device 101 is inputted to a circuit 102 called a CDS (correlative double sampling) circuit and a reset noise is removed there from. After that, the image signal inputted to the CDS circuit 102 is inputted to an offset adder circuit 103 and a predetermined offset voltage is added to the image signal. Then, the offset addition result is inputted to a variable amplifier 104.
The CDS circuit 102 utilizes a predetermined reference voltage VREF inputted by a terminal 106 as a reference of a feed-through part of the image signal, and the variable amplifier 104 constitutes a direct current amplifier which similarly utilizes the reference voltage VREF as a reference of direct current amplification of the image signal.
The variable amplifier 104 is a gain varying unit configured to correct unevenness of an output sensing degree of the CCD 101 and switch a sensitivity setting of the image pickup apparatus. Hereafter, in order to simply and clearly explain about a clamping operation of the image pickup apparatus, the gain is simply handled at a level of only one time.
An amplified output signal from the variable amplifier 104 is, on one hand, inputted to a circuit for processing, recording, and displaying the image (not shown). On the other hand, the amplified output signal from the variable amplifier 104 is inputted to a sample-hold circuit 107 and an OB level sample-held by an OB clamp pulse which is synchronized with a reading timing of an OB pixel inputted from a terminal 109 is inputted to an integration amplifier 105
The integration amplifier 105 has a predetermined integrated time constant configured by a capacitor 105b and a resistance 105c. The integration amplifier 105 has a configuration of a feedback control in which a difference voltage (clamp error voltage) between the sample-held OB level and a predetermined reference voltage VREF inputted from a terminal 106 is integrated in the integrated time constant; and the output (amount of deviation from the voltage VREF (error signal)) is inputted to the offset adder circuit 103 as a subtraction value and is subtracted therein.
FIG. 8 shows a state in which the OB level of an output signal 11C of the variable amplifier 104 is clamped and converged to a current voltage level VREF, and shows an operating waveform of each portion at that time.
A signal waveform 11A is an output waveform of the CDS circuit 102. The signal waveform 11A has an OB pixel output of a predetermined period per one horizontal line read from the imaging device 101. A timing signal which sample-holds a part of the OB pixel output period is an OB clamp pulse 11D.
An OB pixel output of the signal waveform 11A retains a direct voltage relatively close to the reference voltage VREF, by an operation of the CDS (correlative double sampling) circuit 102. However, in actuality, the OB pixel output of the signal waveform 11A has an offset error (of several mV to several tens mV in an ordinary case) because a difference component (CCD offset) between a feed through component and a signal component of the imaging device 101 and a dark current component depending on a temperature of the imaging device are superposed.
The offset error is uneven in each imaging device and fluctuates depending on the temperature. This offset error is amplified by the variable amplifier 104 and is finally outputted as a black level fluctuation VERR of the image signal.
The black level fluctuation VERR is detected and integrated by the sample-hold circuit 107 and the integration amplifier 105 as a difference of the VREF voltage, and is outputted as an integrated output signal 11B (ΔVERR) as shown in FIG. 8, and is subtracted from the output signal 11A of the CDS circuit 102.
By repeating the operation each time an OB clamp pulse 11D is outputted, the integrated output ΔVERR is converged to the black level fluctuation VERR just as shown by the signal waveform 11B, and the OB pixel output of the output signal of the variable amplifier 104 is converged to the reference voltage VREF as shown by the signal waveform 11C.
In the feedback OB clamp circuit like this, an integration time constant in the case of negative feedback of the black level fluctuation VERR by integration is important.
That is, if the time constant is short, follow-up response to the VREF voltage of the output OB level per one horizontal line becomes faster. However, along with that, the amount of fluctuation (frequency of fluctuation) per clamping operation also becomes larger. As a result, a noise in the shape of horizontal stripe can easily occur.
In this regard, the time constant must be set at a predetermined length of time so that the noise in the shape of horizontal stripe does not become a problem in terms of the image quality.
In Japanese Laid-Open Patent Publication No. 2003-143488, the image pickup apparatus for performing clamping processing on the basis of the signal from an optical black region is disclosed.
In the conventional feedback OB clamp circuit like this, the offset error caused by a CCD offset component and a dark current component of the imaging device are considered to be the main cause of the amount of fluctuation of OB. Accordingly, the conventional feedback OB clamp circuit is designed focusing on correcting the offset error components.
Primarily, the offset error component is not so large, being 1/10 or less (several mV to several tens mV) of the full range (up to 1V) of the image signal.
When a strong light such as direct sunlight is incident on the solid-state imaging device such as CCD, a so-called “blooming” phenomenon occurs, that is, electric charges are overflowed in a photoelectric conversion section. When the blooming occurs, there is a case where the overflowed electric charges flow also into the OB pixel portion which is primarily shaded within a photo detection pixel portion and is not dependant on the incident light, and is accumulated therein.
In this case, the OB level cannot be utilized as the correct and precise black reference unlike the offset error component described above. Moreover, the OB level in this case rapidly reaches the level of full range (CCD saturation level VSAT) fluctuation of the image signal in the intense blooming.
FIG. 9 is a view showing the operating waveform of each portion for explaining the clamp operation at the time of occurrence of the blooming. The OB output of the CDS output 11A rapidly reaches the CCD saturation level VSAT as the blooming occurs. The OB output of the CDS output 11A rapidly reaches a CCD saturation level VSAT in accordance with the occurrence of the blooming phenomenon. The OB output of the variable amplifier output 11C is slowly reduced to the reference voltage VREF, in relation to the inaccurate OB level raised to the CCD saturation level VSAT, in a response time with the time constant set by the integration amplifier 105.
Thus, in the configuration of the conventional clamp circuit, there has been a problem in that the image signal follows up the inaccurate black level which deviates substantially from the appropriate OB level when the blooming occurs and that the whole image signal is subject to blacking.
In addition, as a result of following up to the inaccurate black level deviating substantially from the appropriate OB level, it takes very long to return to the normal OB level after the blooming of the imaging device has been cancelled and the blacking continues during the restoration time.